The best replacement for google+ is facebook.

The best alternatives to google+ are rated as following:
★★★★★  facebook
★★  picasa
★★  twitter
★★  flickr
★  linkedin
★  facebook privacy
  blogger
  yammer

Your alternative for google+ is not listed? Suggest!
Speaking about google+, one probably shouldn't forget about youtube.

Other search terms related to google+ and their ratings are:
★★★★★ youtube
★★★★ gmail
★★★ picasa
★★★ gmail account
You have in mind a companion for google+ and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of google+ in all years. Click on graph for more.
google+
facebook
picasa